The present invention relates to combustion gas sensors and in particular to an optical sensor adapted for continuous engine exhaust monitoring.
Accurate, real-time information about the combustion gases and particulates produced by engines is increasingly important in the efficient control of combustion engines such as automotive gasoline engines but also including other mobile and stationary engines such as gas turbine engines. This information can be obtained by spectrographic analysis of combustion products using optical sensors that measure the absorption of light in different frequency bands as the light passes through combustion gases.
While such spectrographic sensors are routinely used in experimental environments, practical use for engine control requires a highly reliable sensor that can operate with minimal maintenance over a long period of time. One significant obstacle to long-term use of such sensors is damage to sensitive optical elements caused by the combustion gases and particulates that can accumulate on the protective optical window of the optical components reducing light transmission and/or creating erroneous readings. One approach to minimizing such fouling and damage is the use of an air curtain that flushes air across an optical window, for example, as described in U.S. Pat. No. 5,592,296 incorporated by reference. In an alternative design taught by U.S. Pat. No. 4,583,859, also incorporated by reference, clean air is flushed along the optical path of the sensor to provide an “air shield”.
A disadvantage to each of these techniques is need for an ample supply of fresh air normally obtained by filtration using filters that must be frequently be cleaned and changed.